This invention relates to remote synchronization of clocks and especially to the synchronization of widely spaced clocks connected by a two-way communication link.
A current method used to synchronize clocks at substantial distances from a master clock is to literally transport a stable clock from the master to the slave in a time so short that the drift of the transported clock is small and then to compare the time of the slave with that of the transported clock. This method presents obvious difficulties when the distance between the clocks is greater than minimal and also requires an additional clock.
Another method depends only upon a reciprocal, time-invariant (within the propagation interval), two-way communication link. This is advantageous where a slave clock must be set quickly and often. A knowledge of the propagation time between master and slave is not required, although the time must remain constant. A system of this type for synchronizing a VHF satellite transponder has been described in an article "Satellite VHF Transponder Time Synchronization" by Jespersen, Kamas, Gatterer and MacDoran, in Proc. IEEE, Vol. 56, No. 7, pp. 1202-1206, July 1968. In the Jespersen et al. method, a voice communication link was maintained between master and slave station. The operator at the slave station merely told the master-station operator to advance or retard the master clock until its tick (pulse) coincided with that of the slave clock.